ESP32: add CPUUsageTracker – core-0 load averages (1/5/15 min)

Track CPU utilisation on core 0 using a FreeRTOS tick hook that
increments per-tick idle/busy counters, sampled every 5s by an
esp_timer callback.  Exponential moving averages with Linux-style
time constants (1 / 5 / 15 min) are maintained in software:

  DECAY1  = exp(-5/60)  ≈ 0.9200  (1-minute window)
  DECAY5  = exp(-5/300) ≈ 0.9835  (5-minute window)
  DECAY15 = exp(-5/900) ≈ 0.9945  (15-minute window)

Core 1 is excluded intentionally: it runs the Arduino loopTask at
100% load for LoRa packet processing, so its figure is always 1.0
and carries no diagnostic value.  All other tasks are pinned to
core 0 by task_pinning.c, so core-0 load reflects the true system
utilisation.

New files:
  arch/esp32/CPUUsageTracker.h   – class declaration
  arch/esp32/CPUUsageTracker.cpp – tick hook + esp_timer sampling

Integration:
  MyMesh::begin() calls _cpu_tracker.begin() on ESP32 builds.
  formatStatsReply() emits "load_avg":[<1m>,<5m>,<15m>] in the
  compact stats JSON payload.
  formatWebStatsSummaryJson() adds the same field to the web-panel
  stats endpoint under core.load_avg.
  The web-panel HUD gains a "Load Avg" metric tile showing all
  three values side-by-side; the core-metrics grid is widened from
  4 to 5 columns.  The tile is rendered conditionally so older
  firmware (or non-ESP32 builds) that omit the field degrade
  gracefully.
  CPUUsageTracker.cpp is added to the esp32_base build_src_filter
  in platformio.ini.

arch/esp32/CPUUsageTracker.cpp

@@ -0,0 +1,51 @@
+#ifdef ESP32
+#include "CPUUsageTracker.h"
+#include "esp_freertos_hooks.h"
+
+volatile uint32_t CPUUsageTracker::s_idle_ticks = 0;
+volatile uint32_t CPUUsageTracker::s_busy_ticks = 0;
+TaskHandle_t      CPUUsageTracker::s_idle_handle = nullptr;
+
+void IRAM_ATTR CPUUsageTracker::s_tick_hook() {   // ← was CPUsageTracker (typo)
+  if (xTaskGetCurrentTaskHandle() == s_idle_handle) {
+    s_idle_ticks++;
+  } else {
+    s_busy_ticks++;
+  }
+}
+
+void CPUUsageTracker::s_sample_cb(void* arg) {
+  static_cast<CPUUsageTracker*>(arg)->_onSample();  // ← was CpuUsageTracker (old name)
+}
+
+void CPUUsageTracker::_onSample() {
+  uint32_t busy = s_busy_ticks;
+  uint32_t idle = s_idle_ticks;
+  uint32_t db = busy - _last_busy;
+  uint32_t di = idle - _last_idle;
+  _last_busy = busy;
+  _last_idle = idle;
+
+  uint32_t total = db + di;
+  float sample = (total > 0) ? (float)db / total : 0.0f;
+
+  _avg1  = DECAY1  * _avg1  + (1.0f - DECAY1)  * sample;
+  _avg5  = DECAY5  * _avg5  + (1.0f - DECAY5)  * sample;
+  _avg15 = DECAY15 * _avg15 + (1.0f - DECAY15) * sample;
+}
+
+void CPUUsageTracker::begin() {
+  s_idle_handle = xTaskGetIdleTaskHandleForCPU(0);
+  esp_register_freertos_tick_hook_for_cpu(s_tick_hook, 0);
+
+  esp_timer_create_args_t args = {
+    .callback        = s_sample_cb,
+    .arg             = this,
+    .dispatch_method = ESP_TIMER_TASK,
+    .name            = "cpu_avg"
+  };
+  esp_timer_create(&args, &_timer);
+  esp_timer_start_periodic(_timer, 5000000ULL);  // 5 seconds
+}
+
+#endif